This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in Japanese Patent Application No. 2001-364638 filed Nov. 29, 2001.
1. Field of the Invention
The present invention relates to an EGR (exhaust gas recirculation) control apparatus for an internal combustion engine, and more particularly to controlling the EGR volume (or EGR rate; same below) by means of an EGR valve and an intake throttle valve.
2. Description of the Related Art
In general, the EGR control in diesel engines controls the EGR volume by controlling the opening of the EGR valve such that the actual EGR volume reaches the target EGR volume corresponding to the running condition of the engine. If the EGR volume is insufficient even when the EGR valve is fully opened, an intake throttle valve is further provided, the intake is throttled in the insufficient region (primarily the low load region of the engine), and the EGR volume is increased.
The value of the intake air volume (new air volume) is generally used as an alternative value to the value of the EGR volume. More specifically, the total intake air volume entering the cylinder is the sum of the intake air volume comprising the new air and the EGR gas volume (EGR volume), and the total in-cylinder intake air volume is substantially constant if the air intake mass or the like. (e.g., boost pressure) is fixed; therefore, controlling the intake air volume makes it possible to control the EGR volume.
When feedback control is conducted for both the EGR valve and the intake throttle valve so that a match with the target EGR volume (or target intake air volume) is established, neither valve can be controlled with correlation or coordination, the valves end up operating arbitrarily, and proper control cannot be achieved. More specifically, the openings of both valves cannot be uniquely determined in relation to a single target value, and control that differs from intended control (for example, both valves close simultaneously) occurs. Therefore, it is believed that feedback control must be provided to one of the valves, and open-loop control must be provided to the other valve.
FIGS. 8 and 9 display examples of conventional control. Displayed herein are examples in which feedback control is provided to the EGR valve, and open-loop control is provided to the intake throttle valve. The value of the EGR volume replaces the value of the intake air (new air) mass, namely, the air mass volume. Duty control is provided to the EGR valve and intake throttle valve, and the opening is changed in accordance with the duty (duty value) of the given duty signal. FIG. 8 illustrates control of the EGR valve, and FIG. 9 illustrates control of the intake throttle valve.
As is shown in FIG. 8, the actual engine speed Ne, fuel injection volume Q, and air mass volume Ga are first read (step 801). During this process, the target air mass volume Gat is calculated in accordance with the map M81 on the basis of the engine speed Ne and fuel injection volume Q (step 802). The map M81 predetermines the relationship between the engine speed Ne or fuel injection volume Q and the air-fuel ratio A/F, and the target air mass volume is calculated from the obtained air-fuel ratio A/F. Next, the deviation between the target air mass volume Gat and the actual air mass volume Ga detected using an air mass sensor is calculated (xcex94Ga=Gatxe2x88x92Ga) (step 803). Based on the deviation xcex94Ga, the proportional gain GP and integral gain GI are calculated from the map (step 804). The drawing displays only the calculation map M82 of the proportional gain GP. Based on this map M82, a high proportional gain GP is incrementally assigned as the deviation xcex94Ga increases. Next, the duty D assigned to the EGR valve is calculated with the aid of the formula D=C (GP+GI) by using the proportional gain GP and the integral gain GI (step 805). C is a specific constant. The actual air mass volume is brought closer to the target air mass volume, and, as a result, the actual EGR volume is brought closer to the target EGR volume by repeating this process flow at specific time intervals.
During the control of the intake throttle valve shown in FIG. 9, the actual engine speed Ne and fuel injection volume Q are read (step 901), and, based on these values, the target valve duty D assigned to the intake throttle valve from the map M91 is calculated (step 902). As the engine speed Ne or fuel injection volume Q increases, the value of the target valve duty D increases on the open side. In this way, the duty assigned to the intake throttle valve is determined without any consideration given to the actual value.
If, however, feedback control is provided only to the EGR valve, and open-loop control is provided to the intake throttle valve in this manner, the tracking of the intake throttle valve becomes poor and controllability suffers when EGR control is provided only to the intake throttle valve in the low-load region of the engine, particularly during excessive operation. Specifically, no feedback of the actual air mass volume is provided, so a discrepancy between the target value and actual value occurs.
Feedback control of both valves is thus preferred, but merely adopting this approach will create a problem of poor controllability because of the uncorrelated control of the two valves, as mentioned above.
In conventional practice, a technique in which feedback control is provided to both valves, control of the EGR valve is stopped (EGR valve OFF) when the EGR valve is nearly fully opened, and the system is switched to control based on the intake throttle valve (intake throttle valve ON) is used in order to address the aforementioned problems. During this simple control switching process, however, the intake throttle valve suddenly moves the moment the system is switched to control based on the intake throttle valve, creating a torque shock from the switch.
With the foregoing problems in view, it is an object of the present invention to ensure adequate controllability and to prevent a switching shock from occurring when one type of control is switched to another in cases in which feedback control is provided to both the EGR valve and the intake vale with regard to EGR control.
The invention of claim 1 is an EGR control apparatus for an internal combustion engine comprising an EGR valve provided to an EGR duct for connecting the intake duct and exhaust duct of an engine; an intake throttle valve provided to the intake duct; feedback control means for providing feedback control to the EGR valve and intake throttle valve such that the actual EGR volume approximates the target EGR volume corresponding to the running condition of the engine; and limiting means for limiting the operable opening ranges of the EGR valve and intake throttle valve in accordance with the target EGR volume.
The invention of claim 2 is the invention according to claim 1 in which the limiting means imposes restrictions so that the openings of the EGR valve and intake throttle valve are variable within specific opening ranges when the target EGR volume falls within a specific range.
The invention of claim 3 is the invention according to claims 1 or 2 in which the limiting means limits the operable opening range of either the EGR valve or the intake throttle valve in accordance with the target opening of the other valve obtained by the feedback.
The invention of claim 4 is the invention according to claim 3 in which the limiting means determines the operable opening ranges of the EGR valve and intake throttle valve in accordance with specific maps.
The invention of claim 5 is the invention according to claims 3 or 4 in which the limiting means imposes restrictions so that the EGR valve remains fully opened when the target opening of the intake throttle valve is equal to or less than a first opening, the EGR valve is operable from the point where the valve is fully opened to the opening where the valve closes as the target opening increases when the target opening is greater than the first opening but is equal to or less than a second opening, and the EGR valve is operable from the point where the valve is fully opened to the point where the valve is fully closed when the target opening is greater than the second opening. The limiting means also imposes restrictions so that the intake throttle valve remains fully opened when the target opening of the EGR valve is equal to or less than a third opening, the intake throttle valve is operable from the point where the intake throttle valve is fully opened to the opening where the intake throttle valve closes as the target opening increases when the target opening is greater than the third opening but is equal to or less than a fourth opening, and the intake throttle valve is operable from the point where the valve is fully opened to the point where the valve is fully closed when the target opening is greater than the fourth opening.
The invention of claim 6 is the invention according to any of claims 1to 5 in which the value of the EGR volume is replaced with the value of the intake air volume entering the intake duct, a means of detecting the intake air volume is provided in the intake duct for the purpose of detecting the actual intake air volume, and the feedback control means determines the target opening of the EGR valve and intake throttle valve on the basis of the deviation between the actual intake air volume and the target intake air volume.
The invention of claim 7 is the invention according to any of claims 1to 6 in which duty control is provided to the EGR valve and intake throttle valve, and the openings of the EGR valve and intake throttle valve are replaced with the duty values of the duty signals assigned thereto.
The invention of claim 8 comprises an EGR valve provided to an EGR duct for connecting the intake duct and exhaust duct of an engine; an intake throttle valve and air mass sensor provided in the intake duct; feedback control means for providing feedback control to the EGR valve and intake throttle valve such that the actual air mass volume detected with the air mass sensor approximates a predetermined target air mass volume in accordance with the running condition of the engine; and valve opening limiting means wherein the range of the duty assigned to either the EGR valve or the intake throttle valve is limited by means of a target duty that corresponds to the other valve and is obtained by the feedback, and restrictions are imposed whereby the openings of the EGR valve and intake throttle valve can be varied within specific ranges with regard to specific regions in which the valves are fully opened.
The invention of claim 9 is an EGR control method for an internal combustion engine in which both an EGR valve provided in an EGR duct communicating with the intake duct of the engine and an intake throttle valve provided in the intake duct undergo feedback control such that the actual EGR volume approximates the target EGR volume corresponding to the running condition of the engine, wherein the operable opening range of one of the EGR valve and the intake throttle valve is mutually limited in accordance with the target opening of the other valve obtained by the feedback.
The invention of claim 10 is the invention according to claim 9 in which the openings of the EGR valve and intake throttle valve can be varied within specific ranges when the target EGR volume falls within a specific range.